1. Field of the Invention
The present invention relates to a DC motor and, more particularly, to a simple-structure, low-vibration DC motor in which cogging is reduced while the performance such as torque is maintained.
2. Description of the Prior Art
In recent years, as the performace of various types of devices such as OA devices are improved, a high-performance (e.g., high torque), low-cogging, low-vibration, high-quality motor is sought for as a DC motor used in such a device. To meet this demand, improvement in inner rotor type DC motors used in these various types of devices is under development as needed. A demand for higher performance such as, for example, a demand for “higher torque” and, on the other hand, a demand for higher quality such as, for example, a demand for “lower cogging and lower vibration” are contradictory, and it is difficult to meet both demands simultaneously.
In an inner rotor type DC motor, a rotor unit is rotatably arranged inside a stator unit, and the rotor unit is rotated by the interaction of the magnetic fluxes of the stator unit and rotor unit.
FIG. 1A is a perspective view of a rotor unit in a conventional inner rotor type DC motor, and FIG. 1B is a view showing the developed pattern of S and N poles formed on the surface of the field magnet of this DC motor by magnetization.
As shown in FIG. 1A, a rotor unit 29 is comprised of a holder 22, a shaft 21 pressed into the holder 22 to be fixed to it and serving as the rotation center, and a field magnet 26 coaxially fixed to the outer surface of the holder 22. The outer surface of the field magnet 26 is magnetized in the circumferential direction at predetermined pitches to form magnetic poles developed pattern as shown in FIG. 1B.
As is apparent from the developed pattern shown in FIG. 1B, S and N poles are alternatively formed by magnetization on the field magnet 26 in the circumferential direction with the same pole width (W1=W2). Consequently, magnetic flux changes in all the magnetic poles occur simultaneously and sharply, causing cogging.
To achieve low cogging, for example, the magnetic poles of the field magnet 26 may be skewed. In this case, a decrease in torque is inevitable. Therefore, this method can be employed only when the torque characteristics have a sufficient margin. In addition, the fabrication of a magnetizing jig is extremely difficult and is accordingly expensive. Also, maintenance of the magnetizing jig is difficult to perform.
Since the skew angle can be obtained only with a cut and try manner, it cannot be obtained easily. Phase correction of the magnetic poles to correspond to a decrease in a harmonic component cannot be performed to decrease the vibration.